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A guanosine to cytosine transversion at position 2 of the fifth intron of the mitochondrial gene COB blocks the ligation step of splicing. This mutation prevents the formation of a base pair within the P1 helix of this group I intron--the RNA duplex formed between the 3' end of the upstream exon and the internal guide sequence. The mutation also reduces the rate of the first step of splicing (guanosine addition at the 5' splice junction) while stimulating hydrolysis at the 3' intron-exon boundary. Consequently, the ligation of exons is blocked because the 3' exon is removed prior to cleavage at the 5' splice junction. The lesion can be suppressed by second-site mutations that preserve the potential for base-pairing at this position. Because the P1 duplex and the P10 duplex (between the guide sequence and the 3' exon) overlap at the affected pairings represent alternative structures that do not, indeed cannot, form simultaneously.

Mutational evidence for competition between the P1 and the P10 helices of a mitochondrial group I intron